1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image by electrostatic printing, electrophotographic printing, or the like and, more particularly, to an image forming apparatus having an intermediate transfer member and a control method.
2. Description of the Related Art
Some conventional image forming apparatuses such as a laser beam printer and digital copying machine form a full-color image by developing electrostatic latent images formed on image carriers (photosensitive drums serving as photosensitive members) with developers (toners) and superposing the developed images (toner images) on an intermediate transfer member or paper, like a full-color image forming apparatus. An image forming apparatus of this type suffers color misregistration. The color misregistration is an image error caused by the geometrical characteristics (e.g., differences in the slope of the scan line in the main scanning direction and the curvature of the scan line) of beams scanned by a plurality of optical scanning mechanisms.
To solve this problem, there has been proposed an image forming apparatus having an adjustment unit which controls to make the geometrical characteristics of scan beams coincide with each other (see Japanese Patent Laid-Open No. 63-271275).
If a scratch or the like exists on an intermediate transfer member (intermediate transfer belt serving as an intermediate transfer member) which receives a toner image formed on the image carrier of the image forming apparatus, a registration mark for checking the amount of color misregistration may not be accurately read when transferred as a toner image. In this case, an amount of color misregistration different from an actual one is detected. As a result, color misregistration cannot be corrected at high precision and may worsen. To solve this problem, there has been proposed a technique of detecting a noise component generated by a scratch or the like on the intermediate transfer belt of an image forming apparatus, and forming a registration mark at a portion other than the noise-generated portion (see Japanese Patent Laid-Open No. 2000-137367).
Another technique has also been proposed (see Japanese Patent Laid-Open No. 2004-325608). In this proposal, a plurality of registration marks are formed on the intermediate transfer belt of an image forming apparatus. The image forming apparatus comprises an abnormality determination unit which determines a registration mark detection abnormality. Color misregistration is corrected on based on detection signals except for a registration mark detection signal determined to be abnormal by the abnormality determination unit.
An image forming apparatus having an intermediate transfer belt or photosensitive belt transfers an image at a predetermined position on paper in the following manner. That is, the timing when the paper conveyance roller conveys paper to a transfer position is determined based on the timing when the optical scanning mechanism starts exposure based on an image signal. Based on the determined timing, an image is transferred from the intermediate transfer belt or photosensitive belt to a predetermined position on paper.
However, this transfer method causes image misregistration until the toner image is transferred onto paper from a position where a toner image is primarily transferred onto the intermediate transfer belt or a position where the photosensitive belt is exposed. The image misregistration occurs owing to variations in belt speed, or a slip between the belt and the belt driving roller.
To solve this problem, the following technique has been proposed (see Japanese Patent Laid-Open No. 11-184348). In this proposal, a registration mark indicating an image position is formed on the photosensitive belt of an image forming apparatus. A detection means for detecting a registration mark is arranged upstream of the transfer position. A paper detection sensor for detecting the leading end of paper is also arranged. By comparing detection timings, misregistration of an image transferred onto paper is detected.
FIG. 14 is a block diagram showing the schematic arrangement of the main part of a conventional image forming apparatus.
In FIG. 14, an image signal output unit 1001 corresponds to an image reading apparatus such as a scanner or an external terminal such as a personal computer. The image signal output unit 1001 outputs an image signal. A mark signal output unit 1002 outputs a mark signal upon reading a registration mark formed on a photosensitive belt 1005 of the image forming apparatus. A sync signal generation unit 1013 outputs an exposure sync signal to the image signal output unit 1001 and mark signal output unit 1002 in synchronism with the image forming timing. Upon receiving the exposure sync signal, the image signal output unit 1001 and mark signal output unit 1002 respectively output an image signal and mark signal to a synthesis unit 1003.
The synthesis unit 1003 synthesizes the received image signal and mark signal, and outputs the synthesized signal to an exposure unit 1004. The exposure unit 1004 modulates the receive signal into a laser driving signal, and irradiates the photosensitive belt 1005 with a laser beam. A mark detection sensor 1008 and paper detection sensor 1009 are arranged near the transfer position of paper P. An image misregistration prediction unit 1012 compares a signal obtained by reading, by the mark detection sensor 1008, the registration mark formed on the photosensitive belt 1005 with a signal obtained by reading the leading end of the paper P by the paper detection sensor 1009. Based on the detection by the sensors 1008 and 1009, the image misregistration prediction unit 1012 can detect an error caused by rotational variations of a photosensitive belt driving roller 1006 or the like.
The following problem arises in the technique of positioning and transferring an image onto paper in the conventional image forming apparatus. A toner image forms a registration mark for detecting the amount of misregistration (so-called color misregistration) between images formed by a plurality of image forming units, similar to a registration mark for determining an image transfer position on paper.
However, the registration mark formed on the photosensitive belt or intermediate transfer belt of the image forming apparatus may not be accurately detected owing to a scratch or the like present on the belt. If the registration mark overlaps the scratch on the belt, the registration mark detection time becomes longer. This results in a failure in accurately detecting a registration mark at the leading end (uppermost-stream registration mark among registration marks formed on the belt in the conveyance direction). When paper conveyance is controlled based on detection of the registration mark at the leading end, the distance from the leading end of paper to the leading end of an image formed on paper varies. In an arrangement in which a detection means is provided upstream of the transfer position to detect a registration mark serving as a reference for controlling paper conveyance, like Japanese Patent Laid-Open No. 11-184348, if the photosensitive belt or intermediate transfer belt is scratched, the detection error increases.
A plurality of registration marks can also be formed on the photosensitive belt or intermediate transfer belt of the image forming apparatus in order to detect the amount of color misregistration. In this case, the registration marks are formed at intact portions on the belt. Alternatively, if the error detection unit detects an error in registration mark detection, the registration mark may not be employed as color misregistration information.
A formed registration mark is paired with an image to determine an image transfer position on paper. The image forming apparatus forms images at predetermined intervals. It is, therefore, very difficult to form a plurality of registration marks for detecting the amount of color misregistration as described above. In addition, the image transfer position may greatly change when the image transfer position on paper is determined directly using a registration mark detection signal.